All of the above, plus:
Keep out of thermal gradients: storage sheds, basements, garages
Keep out of areas of humidity fluctuations (if you live in Hell...sorry....Texas, like I do, this is not easy)
Your best bet is to buy your ivoury intact (I just bought 70 pounds in five pieces), and cut it as you need it. That way, you have less surface area exposed to sunlight, moisture, rats, cats, kids......